1. Field of the Invention
The present invention generally relates to touch-sensitive panels used to implement touch-screen displays. In particular, the present invention relates to touch-sensitive panels that utilize projected capacitive technology to perform touch-sensing functions.
2. Background
Touch-screen displays provide an easy and intuitive interface by which users can control and interact with applications executing on a computer or other electronic device. For example, a touch-screen display may enable a user to select an object or move a cursor rendered to the display by simply touching the display with a finger, stylus or other object. By integrating user input functionality into the display, touch-screen displays enable electronic devices to be operated without a separate input device such as a keyboard, mouse, pointing stick, or touch pad. For at least this reason, touch-screen displays are becoming increasingly popular interfaces for portable and handheld electronic devices, including but not limited to personal digital assistants (PDAs), mobile telephones, and personal media players, in which compactness and ease of use is important. The increased popularity of touch-screen displays may also be attributed to advances in touch-sensing technology that have resulted in both improved performance and reduced cost.
Certain conventional touch-screen displays include a substantially transparent touch-sensitive panel and a controller connected thereto. The touch-sensitive panel is positioned in front of a display, such as a Liquid Crystal Display (LCD) or other type of display, so that the touch-sensitive panel covers the viewable area of the display. The touch-sensitive panel is designed such that it registers a change of state responsive to the touch or proximity of an object such as a finger or stylus. The controller senses the change of state and generates data associated with a touch event responsive thereto. The controller passes the data concerning the touch event to a host processing system which interprets the data as a form of user input.
Various types of technology exist for implementing touch-sensitive panels. These types of technology include resistive, capacitive, infrared, surface acoustic wave, electromagnetic and near field imaging. In a capacitive touch-sensitive panel, an electrostatic field is established around one or more capacitive nodes integrated within the touch-sensitive panel. The proximity or touch of an object, such as an object or stylus, disturbs the electrostatic field around the node(s) in a manner that can be sensed and measured by the controller. Projected capacitive touch-sensitive panels are panels in which the electrostatic field is projected above the surface of the panel, such that the user may not be required to physically touch the touch-screen display in order for a touch to be registered. This provides improved sensitivity and reduced wear and tear on the touch-screen display.
In one type of projected capacitive touch-sensitive panel, the capacitive nodes comprise electrodes formed from a transparent resistive material such as indium tin oxide (ITO). FIG. 1 depicts an example of such a touch-sensitive panel 100, shown using an exploded view. As illustrated in FIG. 1, the capacitive nodes include a series of substantially parallel electrodes 1121-11212 extending in a first direction and a series of substantially parallel electrodes 1221-12220 extending in a second direction, wherein the second direction is substantially orthogonal with respect to the first direction. Each set of electrodes 1121-11212 and 1221-12220 is formed on a corresponding transparent substrate 116 and 126, each of which may comprise for example a polyethylene terephthalate (PET) film or glass. Each set of electrodes 1121-11212 and 1221-12220 is connected to a controller via a corresponding set of conductive traces 1141-11412 and 1241-12420, which may be implemented for example using metal or silver ink. The controller charges each electrode via a corresponding trace, thereby causing an electrostatic field to be projected around the electrode. When a user disturbs the electrostatic field around a particular electrode, the disturbance is sensed by the controller via the trace connected to the electrode.
Taken together, electrodes 1121-11212, conductive traces 1141-11412, and substrate 116 form a first sensor layer 102 of touch-screen panel 100 that is capable of detecting touches in one direction, which may arbitrarily be termed the Y direction. Similarly, taken together, electrodes 1221-12220, conductive traces 1241-12420, and substrate 126 form a second sensor layer 104 of touch-screen panel 100 that is capable of detecting touches in another direction, which may arbitrarily be called the X direction. First sensor layer 102 and second sensor layer 104 are typically laminated together via an intermediate adhesive layer (not shown in FIG. 1) that may also serve to isolate the electrodes of first sensor layer 102 from the electrodes of second sensor layer 104 in an implementation in which the electrodes of one layer are facing the electrodes of the other.
Each electrode of first sensor layer 102 and second sensor layer 104 includes a series of substantially diamond-shaped segments connected by smaller substantially rectangular-shaped segments. When first sensor layer 102 and second sensor layer 104 are properly aligned, the overlap between the substantially diamond-shaped segments of the electrodes of first sensor layer 102 and the substantially diamond-shaped segments of the electrodes of second sensor layer 104 is minimized. This is illustrated in FIG. 2, which depicts a partial blown-up view 200 of touch-sensitive panel 100. This arrangement helps to maximize exposure of each electrode to the surface of touch-sensitive panel 100 while also helping to reduce capacitive coupling between the substantially diamond-shaped segments on the different sensor layers. However, as shown in FIG. 2, the substantially rectangular-shaped portions of the electrodes of first sensor layer 102 and second sensor layer 104 do overlap. Consequently, when there is a voltage differential between an electrode on first sensor layer 102 and an electrode on second sensor layer 102 (e.g., when an electrode on first sensor layer 102 is charged and an electrode on second sensor layer 104 is grounded, or vice versa), parasitic capacitance can form between the overlapping substantially rectangular portions of each electrode. This parasitic capacitance can interfere with proper performance of touch-sensitive panel 100. For example, such parasitic capacitance may cause the controller portion of touch-sensitive panel 100 to interpret an actual touch event as noise or vice versa.
To address this issue, conventional touch-sensitive panel 100 includes an additional layer 106 that is typically laminated to the bottom of second sensor layer 104 and comprises a flat sheet 132 of resistive material such as ITO formed on a substrate 136. In an alternative implementation, flat sheet 132 may be formed on the back of substrate 126 of second sensor layer 104. The flat sheet 132 is also connected to the controller via at least one conductive trace (not shown in FIG. 1). During operation of touch-sensitive panel 100, the controller applies a current to flat sheet 132 in a manner that tends to cancel parasitic capacitance that might develop between overlapping electrodes on first sensor layer 102 and second sensor layer 104. Layer 106 may be referred to in the art as a shield layer. The requirement of including layer 106 to combat parasitic capacitance increases the amount of material required to manufacture touch-sensitive panel 100 as well as the cost.
Another potential problem with the touch-sensitive panel design depicted in FIG. 1 is that internal reflections may occur when light strikes the electrodes on first sensor layer 102 and second sensor layer 104. Such reflections can occur, for example, when ITO used to form the electrodes has a refractive index that is different than an adhesive layer that is used to join first sensor layer 102 to second sensor layer 104. The reflections can make the patterned ITO visible to a user, which is highly undesirable. To address this issue, a particular type of ITO termed index-matched ITO may be used to form the electrodes. The use of such index-matched ITO serves to reduce internal reflections. However, index-matched ITO is typically more expensive than non-index-matched ITO. Furthermore, index-matched ITO is typically only available in a form that has a higher resistance relative to certain other forms of non-index-matched ITO. The use of such relatively high-resistance ITO may require more charge to be used to drive the electrodes of touch sensitive panel 100 and/or may result in reduced sensitivity along portions of the electrodes.
What is needed then is a touch-sensitive panel, such as a projected capacitive touch sensitive panel, for use in a touch-screen display that addresses one or more of the foregoing issues associated with conventional touch-sensitive panels.